1. Field of the Invention
The present invention relates to a method of generating color proof data and a method of generating a color proof in a system for generating a color proof as color pixel images on a sheet according to a density gradation process with an image output device such as a color printer or the like, so that a color proof with a color image formed thereon can be generated for proof reading before a printed color document carrying a half-tone dot image is produced by a color printing machine having rotary presses or the like with press plates mounted thereon.
2. Description of the Related Art
It has heretofore been customary to produce a color proof for examining and correcting colors, etc. before a printed color document which carries a halftone dot image on a final print sheet such as of wood-free paper, coated paper, or art paper is produced by a color printing press.
Color printers are used to produce color proofs because the color printers are relatively simple in structure and inexpensive to manufacture and can produce hard copies with images formed on sheets a plurality of times in a short period of time since, as well known in the art, they do not need the production of process-plate films and presensitized plates which are required by color printing machines.
FIG. 27 of the accompanying drawings shows the sequence of a conventional process of producing a color proof with a color printer.
According to the conventional process of producing a color proof, as shown in FIG. 27, an image on an image document 2 is two-dimensionally read by an image reader such as a color scanner having a CCD linear image sensor or the like, and gradation (continuous tone) image data Ia of each of the colors R (red), G (green), and B (blue) are generated from the read image in a step S1 (image reading process).
Then, the RGB gradation image data Ia are converted by a color conversion process using a color conversion look-up table or the like into halftone-dot area percentage data (also referred to as halftone-dot percentage data or original image pixel halftone-dot percentage data) aj of the four plates of respective colors C (cyan), M (magenta), Y (yellow), and K (black), where j=0, 1, 2, 3 (0 represents the color C, 1 the color M, 2 the color Y, and 3 the color K) in a step S2. The color conversion process has various versions corresponding to different color printing machines, and those versions are based on know-hows of various printing companies depending on their color printing machines.
Images on printed color documents produced by the color printing machines are halftone-dot images. To produce a printed color document actually, halftone-dot area percentage data aj produced by a color conversion process are developed into bit map data, and a process-plate film or the like is generated based on the bit map data. Because an automatic image developing machine is required, a process following the generation of the process-plate film is considerably complex.
In order to facilitate the production of a color proof CPa, a digital color printer (DP) 3 is employed for the reasons described above. The DP 3 forms an image on a donor film by digitally controlling the intensity and time of three-primary emission of LED (light-emitting diode) light or a laser beam per pixel, and transfers the image from the donor film to an image-receiving sheet to form the image thereon. The DP 3 is much more inexpensive than a color printing machine which generates presensitized plates from printing plates and produces a printed color document with the presensitized plates. The DP 3 is also smaller in volume and lighter in weight.
In order to employ the DP 3, it is necessary to convert the halftone-dot area percentage data aj of the four plates C, M, Y, K produced in the step S2 into image data (called "common color space data") independent of devices including a printing device, a CRT, a photographic device, an LET, etc., e.g., tristimulus value data X, Y, Z.
Therefore, the halftone-dot area percentage data aj of the four plates C, M, Y, K are converted into tristimulus value data X, Y, Z by a image data processing in a step S4. The image data processing has heretofore been carried out by a process which uses the Neugebauer's equation.
Prior to the step S4, colorimetric data Xi, Yi, Zi (i represents 2.sup.4 =16 colors for the four plates C, M, Y, K, i=0.about.15) for the colors of printing inks are measured by a calorimeter in a step S3. For measuring the colorimetric data Xi, Yi, Zi, the 16 colors are printed on a print sheet which will be used to produce a printed color document by a color printing machine, thereby producing color patches. Such a printing process is usually called a solid printing process. The 16 colors correspond to the presence and absence of the respective colors C, M, Y, K, which represent a combination of 2.sup.4 =16 colors.
Specifically, the 16 colors include the color W (white) which is present when nothing is printed on the print sheet, the primary colors C, M, Y, the color K (black), and the mixed colors C+M, C+Y, C+K, M+Y, M+K, Y+K, C+M+Y, C+M+K, C+Y+K, M+Y+K, and C+M+Y+K. The colors of reflections from the colors printed on the print sheet are measured by a calorimeter such as a spectrometer, for example, thereby producing the colorimetric data Xi, Yi, Zi.
According to the process which uses the Neugebauer's equation, the colorimetric data Xi, Yi, Zi are multiplied by a coefficient of halftone-dot area percentage data hi (i=0.about.15), as shown below, thus producing the tristimulus value data X, Y, Z in the step S4. EQU X=.SIGMA.hi.multidot.Xi, EQU Y=.SIGMA.hi.multidot.Yi,
and EQU Z=.SIGMA.hi.multidot.Zi (1)
The halftone-dot area percentage data hi (i=0.about.15) of the basic 16 colors in the equations (1) are determined from the halftone-dot area percentage data aj according to probability calculations as follows: EQU h0=(1-c)(1-m)(1-y)(1-k), EQU h1-c.multidot.(1-m)(1-y)(1-k), EQU h2=(1-c).multidot.m.multidot.(1-y)(1-k), EQU h3=c.multidot.m.multidot.(1-y)(1-k), EQU h4=(1-c)(1-m).multidot.y.multidot.(1-k), EQU h5=c.multidot.(1-m).multidot.y.multidot.(1-k), EQU h6=(1-c).multidot.m.multidot.y.multidot.(1-k), EQU h7=c.multidot.m.multidot.y.multidot.(1-k), EQU h8=(1-c)(1-m)(1-y).multidot.k, EQU h9=c.multidot.(1-m)(1-y).multidot.k, EQU h10=(1-c).multidot.m.multidot.(1-y).multidot.k, EQU h11=c.multidot.m.multidot.(1-y).multidot.k, EQU h12=(1-c)(1-m).multidot.y.multidot.k, EQU h13=c.multidot.(1-m).multidot.y.multidot.k, EQU h14=(1-c).multidot.m.multidot.y.multidot.k,
and EQU h15=c.multidot.m.multidot.y.multidot.k (2)
where the halftone-dot area percentage data aj (j=0.about.3) are set to a0=c, a1=m, a2=y, a3=k for an easier intuitive understanding, and c, m, y, k represent existential probability values ranging from 0 to 1.0 converted from (1/255) to which the halftone-dot area percentage data aj of the colors C, M, Y, K are set.
In the equations (2), h3, for example, represents an area percentage of the color C+M, and can be determined by multiplying the probability "c" that the plate of the color C exists, the probability "m" that the plate of the color M exists, the probability (1-y) that the plate of the color Y does not exist, and the probability (1-k) that the plate of the color K does not exist. Therefore, the Neugebauer's equation expressed by the equations (1) can be understood as being an equation based on the theory of probability. The tristimulus value data X, Y, Z thus generated according to the equations (1) are supplied to the DP 3. The DP 3 converts the tristimulus value data X, Y, Z into data of the three primaries with respect to the laser beam or the like, i.e., image data depending on the devices, which may also be called inherent color space data, based on a look-up table (LUT), and thereafter generates the color proof CPa, which is a hard copy with an image formed on a sheet, based on the image data.
In the case where the tristimulus value data X, Y, Z for the DP 3 are generated according to the Neugebauer's equation, the colors of a printed color document to be produced can accurately be reproduced in the image on the hard copy because the colorimetric data measured by a calorimeter as representing the colors of an image to be formed on the printed color document by the color printing machine are employed. However, a peculiar pattern as an image structure caused by interference fringes such as moire, a rosette image, or the like which appears on a printed color document cannot be reproduced in the image on the hard copy.
If such a peculiar pattern actually appears on a printed color document, then it should also accurately be reproduced on a color proof CPa which is outputted by the DP 3. In this respect, the conventional color proof CPa which fails to reproduce a peculiar pattern thereon cannot be said as an accurate proof for a printed color document.
It is believed that no peculiar pattern can be reproduced on a hard copy produced by the DP 3 because the Neugebauer's equation is based on the theory of probability as described above.
For reproducing a peculiar pattern without relying on the process illustrated in FIG. 27, it is necessary for an image output device which outputs a hard copy to have a mechanism, such as a small-size proof printing machine, for producing the same image structure as that of a printed document which is to be approximated. It is difficult and substantially expensive for such a mechanism to cope with all of various printing conditions.
The inventor of the present invention has made extensive research efforts to confirm that a peculiar pattern such as moire, a rosette image, or the like, which is peculiar to a printed document, may accurately be reproduced on a color proof by generating pixel data as input image data for a color printer without relying on the Neugebauer's equation. As a result, the inventor has reached an idea, which serves as a basis for the present invention, for accurately reproducing the colors of a print image and also reproducing a peculiar pattern such as moire, a rosette image, or the like of a halftone-dot image, as disclosed in Japanese laid-open patent publication No. 8-192540.
Another process for reproducing a peculiar pattern such as moire, a rosette image, or the like with a simple device such as a color printer or the like is disclosed as a method of generating proof in U.S. patent application Ser. No. 08/734,018 filed by the applicant of the present application.
According to the disclosed process, the halftone-dot area percentage data of at least three colors including the three primaries are developed into bit map data, and a proof for a printed color document is generated based on the bit map data by a DP or the like. Specifically, while a spatial frequency response peculiar to printed halftone dots of the printed color document is being maintained, an image structure simulation process including a filtering process for cutting off a spatial frequency response inherent in the DP is carried out, and thereafter color shifts caused by the image structure simulation process are corrected, after which color shifts to be caused when the DP is used are corrected.
The proof thus generated is an accurate representation of the colors of a printed color document and an image structure such as moire, a rosette image, or the like (false pattern).
A print sheet of paper on which a color image is formed suffers various types of appearance irregularity. Such visual irregular qualities of paper include "impression irregularity" that is visually recognized as a randomly changing density pattern on a reproduced image which is printed on a print sheet when ink is transferred from a uniform image on a press plate to the print sheet, "harshness" that is visually recognized as a smaller randomly changing density pattern which tends to disturb the edges of a reproduced image which is printed on a print sheet, and "texture" that is visually recognized as a density pattern inherent in the type of paper used as a print sheet. The density pattern "impression irregularity" appears in a longer period than the density pattern "harshness".
The above visual irregular qualities of paper are not taken into account in the DP or the like, and are not reproduced at all on a proof which is outputted from the DP or the like.
One technique for expressing a visual irregular quality uses a transfer film which is being sold as a product by the applicant of the present application. According to this technique, a halftone-dot image formed on a transfer film is transferred under pressure to a print sheet. Although the technique is capable of reproducing "texture" of all the three types of appearance irregularity, it is unable to reproduce the other visual irregular qualities "impression irregularity" and "harshness" which result from the transfer of ink from a press plate to a print sheet of paper.